This invention relates to color cathode ray picture tubes, and is addressed specifically to the manufacture of tubes having shadow masks of the tension foil type in association with a substantially flat face panel. The invention is useful in the manufacture of color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors. It is of special value in the manufacture of cathode ray tubes by the printed screen system.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the face panel. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. As is well known in the art, the shadow mask acts as a color-selection electrode, or "parallax barrier," which ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on assigned phosphor targets. The mask is supported in high tension a predetermined distance from the inner surface of the face panel by one or more support structures known as "rails." The predetermined distance is termed the "Q-height."
By way of example, a 14-inch face panel (diagonal measure) comprises a flat panel of glass that typically has dimensions (in inches) of 10.75 high, 13.5 wide and 0.52 thick; the desired Q-distance is 0.29. Dimensions of the face panel of a tube having a diagonal measure of 19 inches are 14.37 high, 18.25 wide 0.687 inch thick. Q-height is 0.42 inch.
Ideally, the two surfaces of a panel should be perfectly flat and planoparallel insofar as possible. It is of course the inner surface of the panel--the screen-bearing surface--which must ground to a precise degree of flatness. In current production practice, the screen-bearing surface is finished to a flatness of .+-.0.002 inch. This tolerance is adequate for the manufacture of tubes other than the printed-screen system; that is, a system in which a selected shadow mask is used as an optical stencil to deposit phosphors on a specific panel. The panel and the mask are thus "mated," and are fused together when the phosphor deposition is completed.
The process of making a face panel begins with the molding of a glass blank of proper size, but with extra thickness to compensate for the normal warpage that results from pressing and annealing, and the need to finish-grind and lap the blank to the desired thickness and flatness. A gob of molten glass is dropped into a recessed mold surrounded by a ring. A plunger presses the glass to fill the mold. The glass cools and the plunger and ring come apart, and the blank is removed and put on an annealing line. The resulting product is warped, and far out of flat. To provide for grinding and polishing, the blank is made thicker by 10 to 20 percent.
The first step in panel finishing is to rough-grind the blank to approximately the desired thickness, leaving just enough surface glass for finish grinding and lapping. Rough-grinding is accomplished by the rotary face grinding process, using a cupped wheel mounted on a vertical spindle. The work-piece is carried on a revolving table, and as the table revolves, so does the work-piece under the motion of the wheel. While a large quantity of glass is removed rapidly by this method because of the uninterrupted passage of the glass of the panel in contact with a large grinding wheel, the heat generated results in damage to the glass in the form of sub-surface cracks.
The second step is a grinding procedure typically used in grinding optical blanks and is known coloquially as "mud-grinding." The panel is lapped by a rotary, soft cast-iron wheel with the lapping compound comprising a slurry containing progressively finer particles. A relatively small amount of glass is removed--just enough to remove the sub-surface cracks resulting from the first process.
The final grind is also a rotary lapping process known as the free-abrasive machining ("FAM") process. Lapping is accomplished by introducing abrasive particles between a hard steel rotating wheel and the work-piece. The FAM process as applied to face panels is fully described in U.S. Pat. No. 4,884,006 of common ownership herewith. The primary purpose of the FAM process is to provide an internal anti-glare ("IAG") surface on the screening area of the panel; the IAG surface minimizes internal reflections in an operating tube. The FAM process also coincidentally helps to achieve the final flatness requirement, somewhat reducing the lapping time of the second lapping operation. It is noted that the FAM process is used primarily for surface finish. However, in the industry, the process is generally recognized as one intended to achieving a high degree of flatness in a variety of parts and materials.
There remains however a further problem, the solution of which this disclosure is addressed. Typical cathode ray tube panels are large in surface area compared with their thickness, and the modulus of elasticity of the panel glass is about 10 million psi, or about one-third the modulus of elasticity of steel. If the panel is warped or otherwise uneven in surface planarity, it will bend to a flatter position when forced against the wheel of a grinding machine such as the free-abrasive grinding machine. To affect good grinding action within a reasonable time, it is necessary to apply considerable force on the panel, using a pressure pad that serves to distribute the applied force uniformly over the surface of the panel. The panel is then ground flat while the clamping force is applied, and therefore, while the panel is in a bent or stressed condition. As soon as the clamping force is is removed, the panel will "unbend," and the ground surface will spring back toward its original warped condition. (It should be noted that even a firmly clamped panel that is warped can be ground to sub-mil flatness using the free-abrasive machining technique; however, an inordinately long time is required.)
As a result of the problem of panel spring back when the processing load is removed, a maximum flatness of only about .+-.0.002 inch can be achieved in a reasonable time.
While this degree of flatness is adequate for the manufacture of current FTM tubes, it is not suitable for the the manufacture of new generations of FTM tubes in which the screen is applied to the screening area without the use of a mating mask; to achieve this, a panel flatness of .+-.0.0002 inch with respect to a desired planarity is mandatory. A procedure for applying a screen by printing for use with an interchangeable mask system is disclosed in referent copending application Ser. No. 07/654,843, of common ownership. An interchangeable mask system is described and claimed in referent U.S. Pat. Nos. 4,902,257; 4,973,280; and 4,998,901.